Extracorporeal blood oxygenators are widely used to add oxygen to and remove carbon dioxide from a patient's blood during those times when the patient's lungs do not satisfactorily perform this gas exchange function. One example of such a time is during coronary artery bypass graft surgery when the cardiac activity is electively stopped to facilitate the surgery. To perform this function for the lungs, the venous blood is drained from the heart into an extracorporeal oxygenator, oxygenated and returned to the aorta for recirculation throughout the patient's body.
Several types of oxygenators are available. Among these is the membrane oxygenator. A membrane oxygenator, in its basic form, comprises first and second conduits separated by a transfer membrane which is permeable to oxygen and carbon dioxide. During use of the membrane oxygenator, an oxygenating gas is caused to pass through one of the conduits while the patient's blood is caused to flow through the other conduit. Oxygen passes from the oxygenating gas through the transfer membrane and into the blood. Simultaneously, carbon dioxide passes from the blood through the transfer membrane and into the oxygenating gas.
The membrane oxygenator affords communication between the blood and the gas while preventing the actual mixing of the blood and the gas. For example, with a microporous hollow fiber membrane oxygenator, small pores in the walls of the fibers and the highly non-wetting nature of the wall material afford the communication between the blood and the oxygenating gas.
One known way to improve the performance of these membrane oxygenators is to simply provide more membrane surface area. Another known way to improve the performance of membrane oxygenators is to increase the amount of gas transfer per unit of transfer membrane surface area by improving blood mixing over the membrane surface. Some of the highest gas transfer rates for membrane blood oxygenators are believed to be associated with hollow fiber membrane oxygenators as described, for example, in U.S. Pat. Nos. 4,690,758 and 4,735,775. In these oxygenators, the oxygenating gas flows through the hollow fibers and the patient's blood flows around the hollow fibers.
Another known way to improve the performance of membrane oxygenators is to vary the partial pressure difference of the diffusing oxygen and carbon dioxide on opposite sides of the membrane. However, a limiting factor at least with respect to microporous hollow fiber membrane oxygenators is the need to maintain the total pressure of the oxygenating gas at each place within the oxygenator generally at or below the total pressure of the blood opposite the membrane within the oxygenator to avoid bubbling the oxygenating gas into the blood with the attendant risks associated with a gas embolism. Avoidance of the formation of gas bubbles within the blood is complicated by, among other things, the variance of the blood pressure and the variance of the gas pressure within the oxygenator. These pressures are reflective of the differing oxygen and carbon dioxide needs of different patients and the differing needs of a single patient over time. Efforts in the past to maintain the total pressure of the oxygenating gas below that of the blood across the membrane have included simply venting the outlet of the oxygenating gas to atmosphere through a relatively low pressure drop gas path.